The technology of monitoring the condition of pipelines relies on several distinct technologies, such as, for example, pigging, overflight, visual inspection, sensor monitoring such as acoustic monitoring, measurement of potential on cathodic protection systems and many others.
The deployment of distributed fibre-optic sensors on a pipeline has been proposed for the simultaneous measurement of temperature, strain and disturbance affecting the pipeline. The aim of this proposed apparatus is to identify events affecting the pipeline such as leaks, ground movement and third party intervention, as well as frost heave, scouring of the backfill and many others. In addition, commercial systems are available for measuring temperature profiles and fibre-optic disturbance measurements are becoming available which are based either on interferometric arrays or, on a fully distributed basis on coherent Rayleigh backscatter, modalmetric change detection or combined Sagnac/Michelson interferometry.
Typically the disturbance sensors are placed alongside or on the pipeline with the intention of detecting inadvertent or deliberate intrusion on the pipeline. These systems are thus intended to avoid incidents involving often many tens of fatalities which are possible where, for example, an earth moving vehicle hits a pipe or as a result of attempted product theft.
The current invention involves the cooperative operation of arrays (or continua) of acoustic sensors placed on or nearby a pipe of a pipeline with pipe inspection tools or pigs running inside the pipe.
The use of the vibration generated by a pipe inspection tool or pig as a means of determining the rate of progress of the pig has been identified in GB 2 305989, U.S. Pat. No. 4,541,278 and GB 2 394 549. However, in both of these patent applications, all the sensing is within the pig itself. The concept of listening to the pig with continuous sensor arrays outside the pipe is not disclosed. U.S. Pat. No. 5,417,112 proposes installing a pair of geophone-type sensors at discrete locations along the pipe. It also discussed the combination of geophones and magnetometers. U.S. Pat. No. 4,590,799 discusses tracking a pig from signals recorded by a geophone coupled to the pipeline wall at a specified location, near known acoustic features. Going back even further in time U.S. Pat. No. 2,820,959 discusses an electromagnetic transmitter within a pig, used by someone walking along the pipe route to detect a stuck pig. U.S. Pat. No. 5,549,000 discusses acoustic sensors, preferably shear mode geophones, including multiplexed fibre-optic sensors, but all are attached intimately to the pipe.
The current invention provides an accurate and easily operable means to monitor the progress of a pig passing through a pipeline by determining its location and speed of movement.